I doubt that anyone will bother to check the pairs of tourists who, although for the sake of formalities, entered the country separately, but walk and sleep together. I know about the conservative nature of this country, but we all understand that any ideology has a huge amount of hypocrisy - if we turn to religion, then all these "entertainment" games/competitions are not encouraged by the radicals. But nevertheless, Qatar wants to hold this tournament and wants to attract tourists. I am sure that tourists informally will behave very freely during the World Cup, although formal restrictions will allegedly take place.
I agree, to a certain limit. Hotel management can't install cameras inside everyone's room. So if an unmarried couple decide to sleep together, then in 99.99% of the cases there should not be a problem. But the remaining 0.01% is the issue. I am saying this, because previously there have been similar incidents in the GCC. Unmarried couples were arrested and jailed, just because they were having physical relationships. If they really want to attract tourists, then I don't think that they should stick to these stone age laws.